Solar panels convert light to electricity. They are designed to maximize electrical output for each photovoltaic module in a solar panel array and to minimize degradation of the module due to environmental exposure—for instance, from moisture, UV radiation, or oxygen. For that reason, the photovoltaic modules are enclosed within a number of exterior material layers that serve various purposes. Thus, the photovoltaic modules are encapsulated in a protective shell, or encapsulant, of a polymeric material such as ethylene vinyl acetate or another thermoplastic material. The encapsulated photovoltaic modules are then sandwiched between a frontsheet and a backsheet. The frontsheet is designed to allow light to reach the photovoltaic modules. The backsheet is designed to insulate the photovoltaic modules as well as to protect them from mechanical perturbations and environmental exposure.
Generally, the backsheet is constructed from a number of film layers that are laminated together in a thermoforming process. The films used to form the backsheet must meet a number of requirements. Firstly, they should be free of surface defects such as surface pits and particulate residues (or grits) to ensure good layer-to-layer adherence. Secondly, the films should not shrink or wrinkle during the lamination process. Thirdly, the films should be prepared from materials in accordance with various environmental considerations, such as the Waste in Electrical and Electronic Equipment Directive (WEEE) and the Restriction of Hazardous Substances Directive (ROHS). For instance, the films should be halogen-free.
One approach to making films meeting these requirements is suggested by U.S. Pat. No. 7,244,813 titled “Methods of Purifying Polymeric Material.” The material comprises poly(arylene ethers) and poly(alkeny aromatics) designed for use in data storage media applications (e.g., compact disks, blue ray disks and the like). A requirement for such materials is that they contain limited quantities of particulate impurities. Particulate impurities include impurities include gels and carbonized polymeric materials that result during material manufacture. Poly(arylene ether)s and compositions derived therefrom are known to oxidize and form gels if maintained at high temperatures. These resins may also form carbonized “black specks” or degrade in color (darken) if processed at high temperatures for extended periods of time. Even the smallest of these black specks (i.e., specks that are invisible to the human eye) will impede the mechanical performance of films derived from such compositions. Such impurities can be removed by the process of melt-filtration. The exacting performance standards required for materials are met by a melt filtration process disclosed in U.S. Pat. No. 7,244,813, which produces materials that are substantially free of specks that are large enough to be visible to the human eye, as well as specks that are even smaller.
However, the melt filtration process disclosed in U.S. Pat. No. 7,244,813 does not translate well for the preparation of poly(phenylene ether) materials where high processing throughput is a desired goal and the performance standards are not as exacting as for data storage media applications, such as in the preparation of materials used to make photovoltaic backsheet films. One issue is that, because of the extremely small particles that need to be removed, run processing times for the extrusion in U.S. Pat. No. 7,244,813 are short due to performance failures, including the filter plugging, high back pressures, and the like.
As a result, there continues to be a need for processes to make materials destined for use as backsheet films. In particular, there is a need for processes for making halogen-free films that are designed for use in photovoltaic backsheets that have good adherence properties, are free of visible specks or defects, and that can withstand lamination processing conditions.